The Group III-V semiconductor materials such as gallium arsenide, indium phosphide, gallium indium phosphide, indium phosphide arsenide, and gallium indium arsenide phosphide are being utilized for fabrication of various devices such as laser devices or Field Effect Transistors (FET). These devices are formed by a series of deposition processes resulting in a layered structure formed on an essentially single crystal substrate. Generally, a region is introduced within the structure to confine or restrict the flow of current along desired paths, for example, to an active (conductive) region of the device. Various expedients such as a patterned oxide layer or a reverse biased p-n junction are employed for this isolation. However, the oxide layer does not permit epitaxial overgrowth and the p-n junction, while permitting epitaxial overgrowth, yields a structure whose resistivity is highly temperature dependent. Thus, research has been stimulated towards the development of a semi-insulating single crystalline region within the device since the resistivity of such regions may not be strongly temperature dependent and since subsequent overgrowth should be possible.
Semi-insulating material is generally formed by suitably doping the desired Group III-V semiconductor material. For example, in the formation of gallium arsenide based layers for FET applications, one method of forming a semi-insulating gallium arsenide region involves introducing chromium as a dopant. The chromium doped gallium arsenide layer is generally fabricated by chemical vapor deposition (CVD) growth in a gas transport system. In a typical CVD growth procedure, a gallium arsenide wafer is heated and a deposition gas is prepared that includes gallium chloride and arsenic compounds such as As.sub.2 and/or As.sub.4. These materials are transported in a hydrogen stream or in an inert gas stream such as a helium stream. Upon contacting the heated substrate, gallium arsenide is deposited with the release of a chloride containing gas. The reactions involved are shown in the following equations. ##STR1## An appropriate dopant precursor is introduced into the deposition gas stream to produce the desired semi-insulating properties. For example, a chromyl chloride dopant precursor as described in U.S. Pat. No. 4,204,893 issued May 27, 1980 is utilized for producing semi-insulating gallium arsenide. However, chromium compounds are not the only dopant precursors that have been suggested for doping gallium arsenide. Other dopant precursors such as iron pentacarbonyl for gallium arsenide doping have been disclosed. (See U.S. Pat. No. 3,492,175 issued Jan. 27, 1970.)
Indium phosphide has also been formed by a CVD process. In particular, a gas stream including volatile indium halide entities such as InCl, InCl.sub.2, and In.sub.2 Cl.sub.3 and phosphorus containing entities such as PH.sub.3, P.sub.2 and P.sub.4 are utilized in a hydrogen atmosphere to form indium phosphide and HCl as shown in the following equation. ##STR2## Unlike gallium arsenide deposition, an inert gas carrier system such as a helium carrier system does not result in the deposition of indium phosphide. Since the presence of a reducing carrier such as a hydrogen carrier is necessary in conventional CVD growth of indium phosphide, the dopant precursor employed is limited to those that do not undergo reduction to produce elemental metals of low volatility. Premature reduction to a nonvolatile elemental metal by interaction with the carrier gas does not result in dopant incorporation, but instead induces essentially complete depletion of the dopant either in the gas phase or by formation of the metal on the reactor walls. Therefore, until recently, only chromium-based dopant precursors were utilized to form semi-insulating indium phosphide. (See Alferov et al., Soviet Technical Physics Letters, 8(6), 296 (1982) and L. A. Ivanyutin et al. Elektronnaya Tekhnika, No. 6, 155, 20 (1981).)
As disclosed in the article by Alferov et al., supra, chromium-doped indium phosphide epitaxial layers having resistivities of only approximately 5.times.10.sup.3 ohm-cm have been produced. This resistivity level is marginally acceptable as semi-insulating material for discrete devices such as lasers. For arrays of lasers or for integrated circuits, it is highly desirable to have a material with significantly higher resistivity--a resistivity greater than 10.sup.6 ohm-cm--to avoid electrical leakage and undesirable cross coupling of elements in an integrated circuit.
In order to overcome these limitations, indium phosphide having a resistivity up to 1.times.10.sup.9 ohm-cm has been produced utilizing a metal organic chemical vapor deposition (MOCVD) procedure in conjunction with an iron pentacarbonyl or ferrocene based dopant precursor. The use of an iron pentacarbonyl or ferrocene based dopant precursor in the formation of indium phosphide through MOCVD has resulted in device quality semi-insulating layers and has avoided significant loss of dopant through premature deposition of elemental iron. For example, excellent results have been achieved by employing these dopant precursors in conjunction with indium-based organic materials such as alkyl indium-alkyl phosphine adducts, e.g., trimethyl indium-trimethyl phosphine adduct, together with additional phosphine. See J. A. Long et al., J. of Crystal Growth, Vol. 69, pp. 10-14 (1984).
While iron doping of indium phosphide is useful for producing high resistivity, semi-insulating semiconductor material, the resulting material has poor thermal stability. Moreover, since iron is a deep acceptor in indium phosphide and because the semi-insulating material is grown in contact with a p-n junction, the semi-insulating material is susceptible to being rendered conductive in the vicinity of the p-type material because rapidly diffusing p-type impurities such as zinc, cadmium, magnesium, and beryllium change the net carrier concentration from an excess of shallow donors toward an excess of shallow acceptors. This has, in turn, caused the search to continue for other dopants to form semi-insulating indium phosphide. Although a large number of alternate transition metal dopants (Co, Cr, and Mn) have been studied for use with indium phosphide, none has achieved a successful combination of good semi-insulating behavior and thermal stability.
Recently, it was reported that titanium doping of bulk indium phosphide resulted in high resistivity semiconductor material which also exhibited good thermal stability. The semi-insulating bulk crystals were grown by liquid encapsulated Czochralski techniques using pyrolytic boron nitride crucibles. See C. D. Brandt et al., Appl. Phys. Lett., Vol. 48, No. 17, pp. 1162-4 (1986). The high purity titanium source used for liquid encapsulated Czochralski growth is not suited for vapor phase or molecular beam epitaxial growth techniques. Moreover, the results fail to suggest a titanium source suitable for such epitaxial growth techniques which would be capable of producing semi-insulating indium phosphide exhibiting deep donor levels which result from titanium doping as opposed to deep acceptor levels associated with iron doping. While the reported results indicate that titanium doping is more desirable than iron doping in forming semi-insulating indium phosphide, the titanium source and growth techniques applied are incapable of producing or overgrowing semi-insulating indium phosphide epitaxial layers necessary for device fabrication.